1. Field of the Invention
The present invention relates to a method of controlling the microstructures of a Cuxe2x80x94Cr-based contact material for a vacuum interrupter and a contact material manufactured by the method, in which heat-resistant metal elements are added to the Cuxe2x80x94Cr-based contact material in order to obtain an excellent current interrupting characteristic and an excellent voltage withstanding capability.
2. Description of the Related Art
Generally, a vacuum interrupter has excellent current interrupting and voltage withstanding capabilities, long service life, low maintenance cost without frequent repair, a simple structure, small size, environmental compatibility, and inertness from an external environment. Therefore, the vacuum interrupter has been widely used in all kinds of power distribution installation, industrial power installation and intermediate voltage vacuum breakers for national defense, education and science researches. The performance of the vacuum interrupters used for the various purposes is dependent upon an arc characteristic exhibited between the contact surfaces during current interrupting, and the arc characteristic is dependent upon characteristics of contact materials.
Accordingly, the contact material is one of most important factors that determine the performance of vacuum interrupters (For example, see Vol. CHMT-7, pp.25 (1984), The Vacuum Interrupter Contact, IEEE Transaction on Components, Hybrids, and Manufacturing Technology, Paul G. Slade).
The contact materials have to satisfy the following characteristics, in order to execute satisfactorily their function: (1) excellent large current interrupting ability; (2) high voltage withstanding capability; (3) low contact resistance; (4) excellent deposition-resistant characteristic; (5) low amount of consumption (abrasion) of contact; (6) low current chopping value; (7) excellent workability; and (8) sufficient mechanical strength (See U.S. Pat. No. 5,853,083 (1998), issued to Furushawa et al.; U.S. Pat. No. 5,882,488 (1999), entitled xe2x80x9cContact Material for Vacuum Valve and Method of Manufacturing the Same, issued to T. Seki, T. Okutomo, A. Yamamoto, T, Kusano; U.S. Pat. No. 4,870,231 (1989), entitled xe2x80x9cContact for Vacuum Interrupter, issued to E. Naya, M. Okumura; and Vol. 21, No.5, (1993) pp. 447, Contact Material for Vacuum Switching Devices, IEEE Transactions on Plasma Science, F. Heitzinger, H. Kippenberg, Karl E. Saeger, and Karl-Heinz Schroder).
The Cuxe2x80x94Cr-based contact materials for the vacuum interrupters had been developed and manufactured in U.S.A. and U.K. before 1970""s and then have been rapidly extended into Europe and Japan after 1980""s. At present, the Cuxe2x80x94Cr-based contact materials for the vacuum interrupters have been widely used in all over the world. Specifically, until 1980, among the breaker manufacturing companies only four companies of Westinghouse, English Electric, Siemens and Mitsubishi had used the Cuxe2x80x94Cr-based contact material for the commercial purpose. Since 1980s, the characteristics of Cuxe2x80x94Cr-based contact materials have been drastically improved and the Cuxe2x80x94Cr based contact materials have been used in most of commercial intermediate voltage/high current breakers since 1990""s (For example, see Vol. 17, No. 1 (1994) pp. 96, IEEE Transaction on Components, Packaging, and Manufacturing Technology, Paul E. Slade).
Recently, since the application conditions of the contact materials become more complicated and the application range thereof is being extended from an existing cut-off circuit to reactors circuit and electricity storing (capacitor) circuit areas, there is a need to enhance the properties of Cuxe2x80x94Cr-based contact materials that exhibit an excellent current interrupting ability and an high voltage withstanding capability, compared with the existing Cuxe2x80x94Cr based contact materials. In other words, the voltage of the capacitor circuit is twice that of an ordinary circuit, and the restrike of arc in a circuit where an inrush current requiring an excellent high current interrupting characteristic is passed causes a serious problem. To solve these problems, it is necessary to improve the current interrupting and voltage withstanding capabilities in the Cuxe2x80x94Cr-based contact materials.
To improve the properties of the Cuxe2x80x94Cr-based contact materials, the metallic elements such as Mo, W, Nb, Pt, Ta, V and Zr can be added, which results in homogenous microstructes as well as refined Cr particles dispersed in the Cu matrix.
In the conventional manufacturing method for Cuxe2x80x94Cr contact materials, a chromium powder of about 40 xcexcm in average diameter is used to improve the current interrupting and voltage withstanding capabilities of Cuxe2x80x94Cr-based contact material. (See U.S. Pat. No. 5,882,488 (1999), entitled xe2x80x9cContact Material for Vacuum Valve and Method of Manufacturing the Same, issued to T. Seki, T. Okucomo, A. Yamamoto, T, Kusano).
This conventional method is not desirable to obtain Cuxe2x80x94Cr contact materials of high performance with fine grain structures.
The fine chromium powder results in a rise in the production cost of the Cuxe2x80x94Cr-based contact materials and formation of a detrimental tight chromium oxide on the powder surface, which inhibits full densification and causes high oxygen content. In order to manufacture the Cuxe2x80x94Cr-based contact materials with homogeneously fine chromium grain structures from a coarse chromium powder, it is necessary to invent a new technology which allows us to control easily Cuxe2x80x94Cr microstructures.
It is known that if at least one element selected from Mo, W, Ta, Nb, V and Zr is added to the Cuxe2x80x94Cr-based contact material or if the chromium particles in the Cuxe2x80x94Cr-based alloys are fine, the current interrupting and voltage withstanding capabilities of the vacuum breakers is improved. Thus, the fine chromium powder having the average diameter of about 40 xcexcm has been used in the conventional manufacturing process of the Cuxe2x80x94Cr based contact material. This fine Cr powder causes some serious problems in manufacturing the Cuxe2x80x94Cr alloys as well as in the production cost. (See U.S. Pat. No. 5,882,488 (1999), entitled xe2x80x9cContact Material for Vacuum Valve and Method of Manufacturing the Samexe2x80x9d, issued to T. Seki, T. Okutomo, A. Yamamoto, T, Kusano; U.S. Pat. No. 4,870,231 (1989), entitled xe2x80x9cContact for Vacuum Interrupterxe2x80x9d, Issued to E. Naya and M. Okumura; Korean Pat. No. 1609 (1989; Korean Pat. No. 1035 (1993)).
To overcome these problems, there is a need to develop a new technology for microstructure control in manufacturing the Cuxe2x80x94Cr-based contact materials from the coarse chromium powder as a material.
Accordingly, it is an object of the present invention to provide a method of controlling the microstructures of Cuxe2x80x94Cr-based contact materials for vacuum interrupters and contact materials manufactured by the method, in which desirable microstructures are embodied to thereby exhibit the excellent large current interrupting and high voltage withstanding capabilities.
To accomplish this object of the present invention, there is provided a method of controlling the microstructures of Cuxe2x80x94Cr-based contact materials for vacuum interrupters, which comprises the steps of: mixing a copper powder improving electrical characteristics of the contact materials as a matrix material, a chromium powder and a heat-resistant element powder making the chromium particles in the matrix material fine to thereby obtain a mixed powder; and subjecting the mixed powder to one treatment selected from sintering, infiltration and hot pressing to thereby obtain a sintered product.